That is indeed a problem, but there are several factors which I think make it less of a problem than it would be with other "too big to fail" CAs. First, becau

That is indeed a problem, but there are several factors which I think make it less of a problem than it would be with other "too big to fail" CAs. First, because certificate issuance is automated, if Let's Encrypt ever had to migrate to a new root cert they could do so in a way that's almost completely transparent to its clients. Aside from sites that have the old roots pinned, Let's Encrypt could simply start signing new certs with a different root and everyone could just carry on like nothing happened. Second, because of the low validity period on Let's Encrypt certs, if such a transition did become necessary the entire process could be completed in ~90 days. This is in contrast to other "too big to fail" CAs (e.g. Symantec), where the process of distrusting the old roots takes years. These factors don't eliminate the concern entirely, of course, but they do make the overall situation much better.